1. Field of the Invention
The present invention relates to an apparatus for aligning laser beams and more particularly to an apparatus for forming a composite laser beam for use in an image forming apparatus having a scanning means scanning a photosensitive member with the composite beam obtained by combining laser beams from a plurality of laser oscillators and capable of accurately aligning all the laser beams with a reference axis.
2. Description of the Prior Art
There has so far been known laser printers having a scanning means scanning a photosensitive recording member with a composite beam thereby forming a color image. In such laser printers, the optical system has, for example, an arrangement as shown in FIG. 8.
In the optical system, there are provided three, first to third, semiconductor laser oscillators 1, 2, and 3, having different oscillating wavelengths from one another and it is adapted such that laser beams from these oscillators form, on photosensitive recording paper 14, an image of three primary colors (yellow, megenta, and cyan) corresponding to the additive primary colors.
First to third laser beams P.sub.0, Q.sub.0, R.sub.0 with a divergent angle radiated from the first to third laser oscillators 1, 2, 3, respectively, are adapted to be turned into first to third parallel beams P.sub.1, Q.sub.1, R.sub.1, respectively, by means of collimator lenses 4, 5, 6 located ahead.
Further ahead of the first laser oscillator 1, there is disposed a reflecting prism 7 with a reflection angle of 90.degree., of the second laser oscillator 2, there is disposed a combining dichroic prism 8 for reflecting the second parallel beam Q.sub.1 at an angle of 90.degree. and transmitting the first parallel beam P.sub.1 reflected from the reflecting prism 7 thereby combining these beams, and of the third laser oscillator 3, there is disposed a combining dichroic prism 9 for reflecting the third parallel beam R.sub.1 at an angle of 90.degree. and transmitting the combined beam of the first and second parallel beams P.sub.1, Q.sub.1 by means of the reflecting prism 7 and the combining dichroic prism 8 thereby combining these beams.
Thus, these parallel beams P.sub.1, Q.sub.1, R.sub.1 are combined into a composite beam S by means of the reflecting prism 7 and the combining dichroic prisms 8, 9.
The thus formed composite beam S is horizontally deflected through an angle of scan .theta. by means of a polygon mirror 10.
The composite beam S is passed through a lens 11 and reflected downward by a reflecting mirror 12 so as to irradiate, through a window glass 13, the photosensitive recording paper 14 as a member to be irradiated.
Reference numerals 15, 16 and reference numerals 17, 18 each denote rollers for transporting the photosensitive recording paper 14 and reference numeral 19 denotes a mirror for leading a beam to a sensor 20 for detecting the scan starting position of the scanning beam S.
With such an image forming apparatus, it is usual that there occur variations in characteristic and in position of various optical members with the lapse of time and changes in temperature, and in such an event, the composite beam becomes such that is not any more an accurately aligned beam of a plurality of laser beams. Thus, discrepancy between the irradiated points by the beams causes nonregistration of colors.
Therefore, it becomes necessary to accurately align the plurality of laser beams again. The realignment requires equipment for optical axis adjustment and expensive instruments other than the image forming apparatus, and further it requires high skill, and therefore it has been impossible for an ordinary user to perform the alignment.